The present invention relates to a chemical conversion method for forming a phosphate-based chemical conversion film on a substrate made of aluminum or its alloy and an aqueous chemical conversion solution used therefor. It relates more particularly to a chemical conversion method for forming a dense zinc phosphate film on a surface of a substrate made of aluminum or its alloy by using an aqueous, fluoride-free chemical conversion solution and to the aqueous, fluoride-free chemical conversion solution used in the method.
Recently, substrates made of aluminum or its alloy (hereinafter referred to "aluminum substrate") have been used as parts for automobile bodies due to their lightness. Further, the aluminum substrates are widely used as structural members, various parts of machines, and can members, etc. For the purpose of improving their corrosion resistance, etc., the aluminum substrates are generally subjected to a chemical conversion treatment like steel substrates.
Chemical conversion solutions containing phosphates such as zinc phosphate are generally used to form a chemical conversion film on an aluminum substrate. However, since the aluminum substrate originally has a stable oxide layer on its surface, it has been necessary to dissolve this oxide layer before forming a chemical conversion film on the aluminum substrate. For this purpose, fluoride ions have conventionally been introduced into the conventional chemical conversion solutions. In methods using the conventional chemical conversion solutions, without introducing fluoride ions, a zinc phosphate film cannot be formed on the surface of the aluminum substrate.
According to investigation by the inventors, it has been found that the introduction of fluoride ions causes the following reactions on the aluminum substrate:
(1) The natural electrode potential of the aluminum substrate shifts in the negative (cathodic) direction by a great amount, causing the dissolution of the oxide layer of the aluminum substrate and the etching of the aluminum substrate itself, thereby enabling the deposition of the zinc phosphate on the aluminum substrate; and PA1 (2) The corrosion current density of the aluminum substrate in the aqueous chemical conversion solution increases by a great amount. Namely, the fluoride ions etch the surface of the aluminum substrate, whereby a proton reduction reaction (cathodic reaction) is accelerated on the aluminum substrate, thereby increasing a pH value of the aqueous chemical conversion solution around the aluminum substrate, which enables a swift deposition of the zinc phosphate on the aluminum substrate. PA1 (a) etching a surface of the substrate with an aqueous solution of acid or the alkali; PA1 (b) immersing the etched substrate in an aqueous, phosphate-based chemical conversion solution which is substantially free from fluoride ions: and PA1 (c) applying a negative voltage to the substrate during at least a part of the immersion step so that a potential of the substrate reaches a predetermined minimum potential which is lower than a natural electrode potential of the substrate in the aqueous chemical conversion solution.
However, when the chemical conversion treatment is conducted in the presence of fluoride ions, the resulting zinc phosphate film tends to contain a cryolite (Na.sub.3 Al F.sub.6). It is known that the cryolite-containing zinc phosphate film shows reduced adhesion to a paint layer which will be formed thereon. In addition, a chemical conversion treatment causing less environmental pollution problems has been recently desired. In this sense, it has been desired to develop a chemical conversion method wherein fluoride ions are not used at all.
In view of the above requirements, there may be proposed a method in which the naturally occurring aluminum oxide layer of the aluminum substrate is dissolved by an acid or an alkali in advance, and then the chemical conversion treatment is conducted using an aqueous, phosphate-based chemical conversion solution which is free from fluoride ions. By this method, however, a new oxide layer is likely to be formed on the aluminum substrate no sooner than it is immersed in the aqueous, phosphate-based chemical conversion solution, thereby preventing the formation of chemical conversion film of zinc phosphate, etc. on the aluminum substrate.